Research into development of new data storage devices is fueled by the continuing demand for ultra-high information capacity, more data density, and faster readout rates. Conventional data storage techniques rely on storing information bit by bit on the surface of the recording medium. The two-dimensional storage techniques, however, are rapidly reaching their fundamental physical limits beyond which individual bits may be too small to easily inscribe or read. A promising alternative approach is holographic data storage, in which information is stored throughout the volume of the storage medium. The key challenge in the field of holographic data storage is the development of a suitable storage material that meets all the stringent requirements.
Despite intensive research effort, suitable and commercially viable holographic material still remains illusive. In holographic data storage, a complete page of information is recorded as an optical interference pattern created by two intersecting laser beams within a thick photosensitive material. The interference pattern of these two coherent writing beams induces a periodic change in the refractive index of recording material. Though there are a number of photochemical reactions that can be utilized to achieve the refractive index modulation, very few exhibit quantum yields greater than unity. Nonlinearity of the photochemical reaction is essential to the required high sensitivity and hence ultrafast recording speed of the storage media. Photopolymerization is such a nonlinear reaction and hence utilized in fabrication of leading holographic media. However, a number of issues limit the performance and commercial viability of photopolymers. Major issues include shrinkage of the material due to formation of new bonds and diffusion of the monomers, polymerization inhibition due to oxygen and other inhibitor included in formulations to impart long shelf life to the media, induction period and need of pre-exposure due to inhibitors, dynamic range reduction due to pre-exposure, low shelf/archival life and several others. Hence, there is significant unmet need for a high performance holographic media.
Photoinduced isomerization of Dewar benzene has been found to occur in a chain reaction leading to isomerization quantum yields greater than 100 in solution and 1.2-20 in solid state. Moreover, light induced isomerization of dewar benzene is associated with a large change in the geometry and the electronic structure of the molecule and anticipated to result in a high refractive index modulation as a result of photoisomerization. These two properties; a) nonlinearity of photochemical reaction, and b) expected high index modulation, represent an opportunity to develop an information storage material with high recording speed and high capacity. Additionally, the non-diffusive writing process ensure no/insignificant dimensional changes of the media, good thermal stability of dewar benzene reactant guarantee high shelf life, and irreversible isomerization will circumvent fading of the holograms over a long period of time as seen in case of photorefractive polymers, resulting in high archival life of the holographic media. However, so far, materials based on dewar benzene isomerization have failed to exhibit high photosensitivities, easy fabrication process, and high diffraction efficiencies required for holographic data storage (1, 2, 3, 4, 5, 6). To this end, we synthesized various dewar benzene based monomers with varying electronic properties and subsequently investigated information storage properties of the material. Through a systematic study, we found that recording media containing electron deficient dewar benzene monomer co-crosslinked with a traditional crosslinker such as divinyl benzene exhibit excellent storage properties, in the absence of a binder or polymeric matrix, and in the absence of a co-sensitizer.